Field of the Invention
This invention relates in general to a method for calibrating a scanner, and in particular to a method of calibrating a nanometrology device.
Description of the Related Art
Conventional spectrometers typically use dispersive elements to separate light into its spectral components, requiring space and precise alignment of delivery optics. The requirement of holding imaging optics and dispersive optics in precise alignments adds substantially to the size, weight, and assembly costs of optical systems. The size, weight and cost of spectrometers can be reduced by building an integrated optical device on the image sensor. Such an integrated system has further advantages relative to a grating spectrometer in its robust alignment and its sensitivity.
Several types of integrated spectrometers have been demonstrated by using micro-scale Fabry-Perot etalons. A Fabry-Perot etalon is typically made of a transparent medium bounded by two reflecting surfaces to create an optical cavity. The transmission spectrum of the cavity exhibits peaks of transmission corresponding to resonances of the optical cavity. The position of resonances depends very sensitively on the cavity length and the index of the material in the cavity. Required tolerances for cavity length can be of nanometer order, making fabrication challenging.
The standard way to micro-fabricate optically flat steps with nanometer-scale height control is to sequentially etch levels into a dielectric material. Each level is done in a single lithographic and subsequent etching step. There are ways to reduce the number of process steps, such as “combinatorial etching.” Essentially, one needs to perform a square-root of micro-fabrication steps for the overall step amount. Even this approach leaves ˜31 micro-fabrication steps for a structure with 1,000 levels. Because each micro-fabrication step is time consuming and adds costs, minimizing the amount of fabrication steps is desired. For comparison, a typical commercial chip, depending on its complexity, requires 8-32 lithography steps. The resulting fabrication of a 1,000 level structure by digital etching is quite an effort. Other approaches use grey-scale lithography to fabricate steps in a single lithograph step, but the variability of etch processes and material homogeneity usually limit the number of truly distinct levels to around 100-400 levels.
The U.S. federal government has a long history for research in the miniaturization of electronics. Early efforts focused on 2 nm structures and led to superlattice technologies. Programs in the 1980's sought to exploit new tools like “scanning tunneling microscopy” (“STM”) and atomic force microscopy (“AFM”). STM and AFM are now standard tools for today's nanotechnology.
Since the invention of the STM by IBM in 1982, a family of similar techniques known as scanning probe microscopy (“SPM”) has developed quickly into a powerful tool for surface and nano-science. Among these techniques is AFM, introduced in 1986. Similar to STM, AFM uses a sharp probe that scans the sample in a raster-fashion to detect changes in surface properties such as topography and charge distribution. Unlike STM, AFM does not require conductive samples; rather than measuring the tunneling current, AFM senses the interactive forces (Van der Waals, electrostatic, and magnetic forces, for example) that range from 10−9 to 10−6 N, between the tip and the sample surface. Excellent three-dimensional spatial resolution (with sub-nanometer scales) can be achieved, in contrast to other forms of microscopy that typically can offer only high resolution in lateral dimensions. However, measuring step speed on AFM images proves difficult due to spatial uncertainties associated with the electronics and mechanics of AFM. A common problem for sequential AFM imaging is the thermal drift of piezos that causes the scanner to move away from the original field of view. Hence, standards and references are required for proper operation of an AFM and to verify optimal operating conditions and calibration of the instrument. Standards are used to assure that the absolute measurements are correct, while references assure that the instrument is giving consistent results. For establishing calibrated topography measurements, the AFM scanner must be certified with calibration standards having pre-established dimensions in the X, Y and Z axis. References can be used for establishing that an AFM mode is operating correctly and for establishing the proper operation of the AFM. It is generally not difficult to obtain accurate XY calibration references. It is, however, much more difficult to obtain accurate Z-axis results. It is difficult to control Z piezo dynamics because, during scanning the X- and Y-axes move at a constant rate whiles the Z axis does not.
Calibration standards/references are needed to calibrate AFM in the vertical axis. For calibrations greater than 10 nm step height, standards or references are typically fabricated by etching patterns in a quartz substrate. Another type of reference is etched silicon, or silicon dioxide coated with a uniform layer of metal. When calibrating the instrument for Z height measurements below ˜10 nm, nano-spheres, atomic terraces of silicon may be used as a reference specimen.
Height measurements in an AFM require that the piezo-electric ceramics in the Z axis of the microscope be both linear and calibrated. Due to the high cost of producing calibration standards, the microscope is often calibrated at only one height. However, if the relationship between the measured Z height and the actual Z height is not linear, then the height measurements will not be correct.
Process Specialties (“PSI”) USA, Inc. sells a so-called “New Dual Thickness Technology” calibration reference. PSI's calibration reference, as its name implies, is limited to two heights for the determination of the linearity of metrology tools over a specific thickness range without the need to load and set-up a second standard.